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1. International Journal of Engineering Inventions
ISSN: 2278-7461, www.ijeijournal.com
Volume 1, Issue 6 (October2012) PP: 32-39
Constant Power Control of Dfig Wind Turbines
With Supercapacitor Energy Storage
A.Ramana1, D.Chinnakullay reddy2
M.Tech, Madanapalle Institute of Technology & Science
M.Tech; Asst.Professor, Madanapalle Institute of Technology & Science
Abstract––With the increasing penetration of wind power into electric power grids, energy storage devices will be
required to dynamically match the intermittency of wind energy. This paper proposes a novel two-layer constant power
control scheme for a wind farm equipped with doubly fed induction generator (DFIG) wind turbines. Each DFIG wind
turbine is equipped with a super capacitor energy storage system (ESS) and is controlled by the low-layer wind turbine
generator (WTG) controllers and coordinated by a high-layer wind farm supervisory controller (WFSC). The WFSC
generates the active power references for the low-layer WTG controllers according to the active power demand from or
generation commitment to the grid operator; the low-layer WTG controllers then regulate each DFIG wind turbine to
generate the desired amount of active power, where the deviations between the available wind energy input and desired
active power output are compensated by the ESS. Simulation studies are carried out in PSCAD/EMTDC on a wind farm
equipped with 15 DFIG wind turbines to verify the effectiveness of the proposed control scheme.
Index Terms––Constant power control (CPC), doubly fed induction generator (DFIG), energy storage, supervisory
controller, wind turbine.
I. WIND TURBINE
Generators (WTGs) are usually controlled to generate maximum electrical power from wind under normal wind
conditions. However, because of the variations of the wind speed, the generated electrical power of a WTG is usually
fluctuated. Currently, wind energy only provides about 1%–2% of the U.S.’s electricity supply. At such a penetration level, it
is not necessary to require WTGs to participate in automatic generation control, unit commitment, or frequency regulation.
However, it is reasonable to expect that wind power will be capable of becoming a major contributor to the nation’s and
world’s electricity supply over the next three decades. For instance, the European Wind Energy Association has set a target
to satisfy more than 22% of European electricity demand with wind power by 2030 [1]. In the U.S., according to a report [2]
by the Department of Energy, it is feasible to supply 20% of the nation’s electricity from wind by 2030. At such high levels
of penetration, it will become necessary to require WTGs to supply a desired amount of active power to participate in
automatic generation control or frequency regulation of the grid [3]. However, the intermittency of wind resources can cause
high rates of change (ramps) in power generation [4], which is a critical issue for balancing power systems. Moreover, to
optimize the economic performance of power systems with high penetrations of wind power, it would be desired to require
WTGs to participate in unit commitment, economic dispatch, or electricity market operation [5]. In practice, short-term wind
power prediction [6] is carried out to help WTGs provide these functions. However, even using the state-of-the-art methods,
prediction errors are present [5]. Under these conditions, the replacement power is supported by reserves, which, however,
can be more expensive than base electricity prices [7]. To enable WTGs to effectively participate in frequency and active
power regulation, unit commitment, economic dispatch, and electricity market operation, energy storage devices will be
required to dynamically match the intermittency of wind energy. In [8], the authors investigated and compared different
feasible electric energy storage technologies for intermittent renewable energy generation, such as wind power. Currently,
pumped water and compressed air are the most commonly used energy storage technologies for power grids due to their low
capital costs [9]. However, these two technologies are mheavily dependent on geographical location with relatively low
round-trip efficiency. Compared with their peers, batteries and supercapacitors are more efficient, have a quicker response to
demand variations, and are easy to develop and ubiquitously deployable. Compared to batteries, supercapacitors have a
higher power density, higher round-trip efficiency, longer cycle life, and lower capital cost per cycle [10]. Therefore,
supercapacitors are a good candidate for short-term (i.e., seconds to minutes) energy storage that enables WTGs to provide
the function of frequency regulation and effectively participate in unit commitment and electricity market operation. The use
of supercapacitors [10] or batteries [11]–[13] as energy storage devices for WTGs has been studied by some researchers.
However, these studies only focused on control and operation of individual WTGs and did not investigate the issues of
WTGs to participate in grid regulation. This paper proposes a novel two-layer constant power control (CPC) scheme for a
wind farm equipped with doubly fed induction generator (DFIG) wind turbines [14], where each WTG is equipped with a
supercapacitor energy storage system (ESS).
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2. Constant Power Control of Dfig Wind Turbines with Supercapacitor Energy Storage
Fig. 1. Configuration of a DFIG wind turbine equipped
with a supercapacitor ESS connected to a power grid.
The CPC consists of a high-layer wind farm supervisory controller (WFSC) and low-layer WTG controllers. The
highlayer WFSC generates the active power references for the lowlayer WTG controllers of each DFIG wind turbine
according to the active power demand from the grid operator. The low-layer WTG controllers then regulate each DFIG wind
turbine to generate the desired amount of active power, where the deviations between the available wind energy input and
desired active power output are compensated by the ESS. Simulation studies are carried out in PSCAD/EMTDC for a wind
farm equipped with 15 DFIG wind turbines to verify the effectiveness of the proposed control scheme.
II. DFIG WIND TURBINE WITH ENERGY STORAGE
Fig. 1 shows the basic configuration of a DFIG wind turbine equipped with a super capacitor-based ESS. The low-
speed wind turbine drives a high-speed DFIG through a gearbox. The DFIG is a wound-rotor induction machine. It is
connected to the power grid at both stator and rotor terminals. The stator is directly connected to the grid, while the rotor is
fed through a variable-frequency converter, which consists of a rotor-side converter (RSC) and a grid-side converter (GSC)
connected back to back through a dc link and usually has a rating of a fraction (25%–30%) of the DFIG nominal power. As a
consequence, the WTG can operate with the rotational speed in a range of ±25%–30% around the synchronous speed, and its
active and reactive powers can be controlled independently. In this paper, an ESS consisting of a super capacitor bank and a
two-quadrant dc/dc converter is connected to the dc link of the DFIG converters. The ESS serves as either a source or a sink
of active power and therefore contributes to control the generated active power of the WTG. The value of the supercapacitor
bank can be determined by
Cess=2PnT/V2SC (1)
where Cess is in farads, Pn is the rated power of the DFIG in watts, VSC is the rated voltage of the supercapacitor bank in
volts, and T is the desired time period in seconds that the ESS can supply/store energy at the rated power (Pn) of the DFIG.
The use of an ESS in each WTG rather than a large single central ESS for the entire wind farm is based on two reasons.
First, this arrangement has a high reliability because the failure of a single ESS unit does not affect the ESS units in other
WTGs. Second, the use of an ESS in each WTG can reinforce the dc bus of the DFIG converters during transients, there by
enhancing the low-voltage ride through capability of the WTG [10].
Fig. 2. Overall vector control scheme of the RSC.
III. CONTROL OF INDIVIDUAL DFIG WIND TURBINE
The control system of each individual DFIG wind turbine generally consists of two parts: 1) the electrical control
of the DFIG and 2) the mechanical control of the wind turbine blade pitch angle [14], [15] and yaw system. Control of the
DFIG is achieved by controlling the RSC, the GSC, and the ESS (see Fig. 1). The control objective of the RSC is to regulate
the stator-side active power Ps and reactive power Qs independently. The control objective of the GSC is to maintain the dc-
link voltage Vdc constant and to regulate the reactive power Qg that the GSC exchanges with the grid. The control objective
of the ESS is to regulate the active power Pg that the GSC exchanges with the grid. In this paper, the mechanical control of
the wind turbine blade pitch angle is similar to that in [15].
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3. Constant Power Control of Dfig Wind Turbines with Supercapacitor Energy Storage
A. Control of the RSC
Fig. 2 shows the overall vector control scheme of the RSC, in which the independent control of the stator active
power Ps and reactive power Qs is achieved by means of rotor current regulation in a stator-flux-oriented synchronously
rotating reference frame [16]. Therefore, the overall RSC control scheme consists of two cascaded control loops. The outer
control loop regulates the stator active and reactive power independently,which generates the reference signals i∗ dr and i∗
qr of the d- and q-axis current components, respectively, for the inner-loop current regulation. The outputs of the two current
controllers are compensated by the corresponding cross-coupling terms vdr0 and vqr0 [14], respectively, to form the total
voltage signals vdr and vqr. They are then used by the pulse width modulation (PWM) module to generate the gate control
signals to drive the RSC. The reference signals of the outer-loop power controllers are generated by the high-layer WFSC.
Fig. 3. Overall vector control scheme of the GSC.
Fig. 4. Configuration and control of the ESS.
B. Control of the GSC
Fig. 3 shows the overall vector control scheme of the GSC, in which the control of the dc-link voltage Vdc and the
reactive power Qg exchanged between the GSC and the grid is achieved by means of current regulation in a synchronously
rotating reference frame [16]. Again, the overall GSC control scheme consists of two cascaded control loops. The outer
control loop regulates the dc-link voltage Vdc and the reactive power Qg, respectively, which generates the reference signals
i∗ dg and i∗ qg of the d- and q-axis current components, respectively, for the inner-loop current regulation. The outputs of
the two current controllers are compensated by the corresponding cross coupling terms vdg0 and vqg0 [14], respectively, to
form the total voltage signals vdg and vqg. They are then used by the PWM module to generate the gate control signals to
drive the GSC.
The reference signal of the outer-loop reactive power controller is generated by the high-layer WFSC.
C. Configuration and Control of the ESS
Fig. 4 shows the configuration and control of the ESS. The ESS consists of a supercapacitor bank and a two-
quadrant dc/dc converter connected to the dc link of the DFIG. The dc/dc converter contains two insulated-gate bipolar
transistor (IGBT) switches S1 and S2. Their duty ratios are controlled to regulate the active power Pg that the GSC
exchanges with the grid. In this configuration, the dc/dc converter can operate in two different modes, i.e., buck or boost
mode, depending on the status of the two IGBT switches. If S1 is open, the dc/dc converter operates in the boost mode; if S2
is open, the dc/dc converter operates in the buck mode. The duty ratio D1 of S1 in
the buck mode can be approximately expressed as D1=VSC/Vdc (2)
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4. Constant Power Control of Dfig Wind Turbines with Supercapacitor Energy Storage
Fig. 5. Blade pitch control for the wind turbine.
The operating modes and duty ratios D1 and D2 of the dc/dc converter are controlled depending on the relationship
between the active powers Pr of the RSC and Pg of the GSC. If Pr is greater than Pg, the converter is in buck mode and D1
is controlled, such that the supercapacitor bank serves as a sink to absorb active power, which results in the increase of its
voltage VSC. On the contrary, if Pg is greater than Pr, the converter is in boost mode and D2 is controlled, such that the
supercapacitor bank serves as a source to supply active power, which results in the decrease of its voltage VSC. Therefore,
by controlling the operating modes and duty ratios of the dc/dc converter, the ESS serves as either a source or a sink of
active power to control the generated active power of the WTG. In Fig. 4, the reference signal P∗ g is generated by the high-
layer WFSC.
D. Wind Turbine Blade Pitch Control
Fig. 5 shows the blade pitch control for the wind turbine, where ωr and Pe (= Ps + Pg) are the rotating speed and
output active power of the DFIG, respectively. When the wind speed is below the rated value and the WTG is required to
generate the maximum power, ωr and Pe are set at their reference values, and the blade pitch control is deactivated. When
the wind speed is below the rated value, but the WTG is required to generate a constant power less than the maximum
power, the active power controller may be activated, where the reference signal P∗ e is generated by the high-layer WFSC
and Pe takes the actual measured value. The active power controller adjusts the blade pitch angle to reduce the mechanical
power that the turbine extracts from wind. This reduces the imbalance between the turbine mechanical power and the DFIG
output active power, thereby reducing the mechanical stress in the WTG and stabilizing the WTG system. Finally, when the
wind speed increases above the rated value, both ωr and Pe take the actual measured values, and both the speed and active
power controllers are activated to adjust the blade pitch angle.
IV. WIND FARM SUPERVISORY CONTROL
The objective of the WFSC is to generate the reference signals for the outer-loop power controllers of the RSC and
GSC, the controller of the dc/dc converter, and the blade pitch controller of each WTG, according to the power demand from
or the generation commitment to the grid operator. The implementation of the WFSC is described by the flowchart in Fig. 6,
where Pd is the active power demand from or the generation commitment to the grid operator; vwi and Vessi are the wind
speed in meters per second and the voltage of the supercapacitor bank measured from WTG i (i = 1, . . . , N), respectively;
and N is the number of WTGs in the wind farm. Based on vwi, the optimal rotational speed ωti,opt in radians per second of
the wind turbine can be determined, which is proportional to the wind speed vwi at a certain pitch angle βi
ωti, opt = k(βi)vwi (3)
where k is a constant at a certain value of βi. Then, the maximum mechanical power Pmi,max that the wind turbine extracts
from the wind can be calculated by the well-known wind turbine aerodynamic characteristics
Pmi,max =1/2ρiArv3wiCPi(λi,opt, βi) (4)
where ρi is the air density in kilograms per cubic meter; Ar = πR2 is the area in square meters swept by the rotor blades, with
R being the blade length in meters; and CPi is the power coefficient, which is a function of both tip-speed ratio λi and the
blade pitch angle βi, where λi is defined by
λi = ωtiR/vwi. (5)
In (4), λi,opt is the optimal tip-speed ratio when the wind turbine rotates with the optimal speed ωti,opt at the wind speed
vwi.
Given Pmi,max, the maximum active power Pei,max generated by the WTG can be estimated by taking into account the
power losses of the WTG [14]
Pei,max = Pmi,max − PLi = Psi,max + Pri,max (6)
where PLi is the total power losses of WTG i, which can be estimated by the method in [14]; Psi,max and Pri,max are
the maximum DFIG stator and rotor active powers of WTG i, respectively. In terms of the instantaneous variables in Fig. 1,
the stator active power Ps can be written in a synchronously rotating dq reference frame [16] as follows:
Ps =3/2(vdsids + vqsiqs)
≈ 3/2[ωsLm(iqsidr − idsiqr) + rs(i 2ds + I2 qs)] (7)
where vds and vqs are the d- and q-axis voltage components of the stator windings, respectively; ids and iqs are the stator d-
and q-axis current components, respectively; idr and iqr are the rotor d- and q-axis current components, respectively; ωs is
the rotational speed of the synchronous reference frame; and rs and Lm are the stator resistance and mutual inductance,
respectively. Similarly, the rotor active power is calculated by
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5. Constant Power Control of Dfig Wind Turbines with Supercapacitor Energy Storage
Pr =3/2(vdridr + vqriqr)
≈ 3/2[sωsLm(iqsidr − idsiqr) + rr[I 2dr + I 2qr](8)
where vdr and vqr are the d- and q-axis voltage components of the rotor windings, respectively; s is the slip of the DFIG
defined by
s = (ωs − ωr)/ωs (9)
where ωr is the DFIG rotor speed. (7) and (8) yield
s = −(Pr − 3i2 rr)(/Ps − 3i 2srs) (10)
where is =(I 2ds + I 2qs/2)1/2 and
ir =(I 2dr + I 2qr/2)1/2.
If neglecting the stator copper loss 3i 2s rs and rotor copper loss 3i 2r rr of the DFIG, the relationship between the stator and
rotor active powers can be approximated by Pr = -sPs. (11)
According to (6) and (10) [or (11)], Psi,max and Pri,max of each WTG can be determined. Then, the total maximum
mechanical power Pm,max, DFIG output active power Pe,max, and stator active power Ps,max of all WTGs in the wind
farm can be calculated as
In order to supply constant power Pd to the grid, the deviation Pess,d between the demand/commitment Pd and the
maximum generation Pe,max is the power that should be stored in or supplied from the ESSs of the WTGs Pess,d = Pe,max
− Pd.
On the other hand, the capability of each ESS to store or supply power depends on the capacitance Cess and the voltage
Vessi of the supercapacitor bank. During normal operation, Vessi must be maintained within the following range: Vi,min <
Vessi < Vi,max (16)
where Vi,max and Vi,min are the maximum and minimum operating voltages of the supercapacitor bank, respectively. The
maximum power Pessi,max that can be exchanged between the supercapacitor bank and the DFIG dc link of WTG i can be
determined by
Fig. 8. Proposed two-layer CPC scheme for the wind farm.
Fig. 9. Configuration of a wind farm equipped with 15 DFIG wind turbines connected to a power grid.
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6. Constant Power Control of Dfig Wind Turbines with Supercapacitor Energy Storage
V. SIMULATION RESULTS
Simulation studies are carried out for a wind farm with 15 DFIG wind turbines (see Fig. 9) to verify the
effectiveness of the proposed control scheme under various operating conditions. Each DFIG wind turbine (see Fig. 1) has a
3.6-MW power capacity [14], [15]. The total power capacity of the wind farm is 54 MW. Each DFIG wind turbine is
connected to the internal network of the wind farm through a 4.16/34.5-kV voltage step-up transformer. The high-voltage
terminals of all transformers in the wind farm are connected by 34.5-kV power cables to form the internal network of the
wind farm. The entire wind farm is connected to the utility power grid through a 34.5/138-kV voltage step-up transformer at
PCC to supply active and reactive powers of P and Q, respectively. In this paper, the power grid is represented by an infinite
source. The ESS of each WTG is designed to continuously supply/store 20% of the DFIG rated power for approximately 60
s. Then, the total capacitance of the supercapacitor bank can be obtained from (1). The parameters of the WTG, the ESS, and
the power network are listed in the in the Appendix. Some typical results are shown and discussed in this section.
A.CPC During Variable Wind Speed Conditions
Fig. 10 shows the wind speed profiles of WTG1 (vw1), WTG6 (vw6), and WTG11 (vw11). The wind speeds across
the three WTGs vary in a range of ±3 m/s around their mean value of 12 m/s. The variations of wind speed cause
fluctuations of the electrical quantities of the WTGs. As shown in Fig. 11, if the wind farm is not equipped with any energy
storage devices or the proposed CPC scheme, the wind speed variations in the wind farm result in significant fluctuations of
the total output active power at the PCC. The wind farm power output deviates significantly from the active power demand
or commitment. In future electric power grids where the penetration of wind power is high (e.g., 20%), such active power
fluctuations can bring severe problems to grid operation.
Fig. 10. Wind speed profiles of WTG1, WTG6, and WTG11.
Fig. 11. Comparison of the wind farm power output and the constant power demand from or commitment to the grid
operator: Without ESSs and the proposed CPC scheme.
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7. Constant Power Control of Dfig Wind Turbines with Supercapacitor Energy Storage
Fig. 12. Comparison of the wind farm power output
(measured at PCC) andthe constant power demand from
or commitment to the grid operator: WithESSs and the
proposed CPC scheme.
Fig. 12 compares the total output active power of the wind farm with the power demand from or commitment to the grid
operator, where each WTG is equipped with an ESS as shown in Fig. 1. The ESS stores energy when the WTG generates
more active power than the demand/commitment and supplies energy when the WTG generates less active power than the
demand/commitment. The resulting output power of the wind farm is therefore controlled at a constant value as required by
the grid operator.
Fig. 13. Active powers of all WTGs and the wind farm.
Fig. 14. Stator active power (Ps1), GSC active power (Pg1), and total active power output (Pe1) of WTG1.
Fig. 15. Rotor active power (Pr1) and active power stored in or supplied bythe ESS (Pess1) of WTG1.
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8. Constant Power Control of Dfig Wind Turbines with Supercapacitor Energy Storage
Fig. 13 shows the total stator active power Ps and the total GSC active power Pg of all WTGs, as well as the total output
active power P (measured at PCC) of the wind farm. Through the control of the proposed CPC scheme, the variations of the
stator active power are exactly compensated by the variations of the GSC active power. Consequently, the total output active
power of the wind farm is constant. However, the total output active power Pei of each individual WTG, which is the sum of
the stator active power Psi and the GSC active power Pgi, is usually not constant, as shown in Fig. 14 for WTG 1. The
deviations between the RSC active power (Pri) and the GSC active power (Pgi) of each WTG are stored in or supplied by
the ESS (Pessi), as shown in Fig. 15 for WTG 1.
Fig. 16. Voltages of the supercapacitor banks of WTG1,WTG6, andWTG11
VI. CONCLUSION
This paper has proposed a novel two-layer CPC scheme for a wind farm equipped with DFIG wind turbines. Each
wind turbine is equipped with a supercapacitor-based ESS, which isconnected to the dc link of the DFIG through a two-
quadrant dc/dc converter. The ESS serves as either a source or a sink of active power to control the generated active power
of the DFIG wind turbine. Each individual DFIG wind turbine and its ESS are controlled by low-layer WTG controllers,
which are coordinated by a high-layer WFSC to generate constant active power as required by or committed to the grid
operator. Simulation studies have been carried out for a wind farm equipped with 15 DFIG wind turbines to verify the
effectiveness of the proposed CPC scheme. Results have shown that the proposed CPC scheme enabled the wind farm to
effectively participate in unit commitment and active power and frequency regulations of the grid. The proposed system and
control scheme provides a solution to help achieve high levels of penetration of wind power into electric power grids.
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